Air Gap Required in Roof as Want to Fill With PIR Boards for Extra Insulation? Information, help, tips and advice on cavity walls, ceilings and lofts etc....

[smallpaul]

We have a sloping roof which comes down over our staircase. I have discovered that some of this doesn't have any insulation in it, and other bits of it don't have enough! The rafters are only 100mm, so not a huge amount of room to add lots of insulation between the rafters. It is an old house, and there is no roofing felt. So, question is, would it be ok to fill the depth of the rafters with PIR board, so that it is up to the tile battens? As there is no roofing felt, any vapour wouldn't be trapped there! I am also planning on putting some insulation on the underside of the rafters, and sealing joints in insulation with metalised tape, which would minimise vapour getting through anyway! Thoughts?

[welsh brickie]

there is an insulating product called triso super 10, its extremely thin layer of insulation foil that you can staple to your joists and then plasterboard over, leaving the gap between the joists free.
Equal to 210mm of ordinary fibreglass insulation, I think this is your best bet